1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer, an optical integrated circuit and a light transceiver using the same.
2. Description of the Prior Art
FIG. 1 is a plan view showing the structure of a conventional optical multiplexer/demultiplexer. In this optical multiplexer/demultiplexer 1, a main core including a core 3 and a core 4 is formed on a tabular cladding layer 2. One end of the core 3 and one end of the core 4 are optically connected to each other through a slit 5 cut in the cladding layer 2. The ends of the main core are linear in shape, and the central portion of the main core is curved in the shape of S. The slit 5 is formed in the curved portion of the main core. Also, a core 6 branching from the core 3 at the position of the slit 5 is formed on the same side as the core 3. A thin-film filter 7 is inserted in the slit 5.
In this optical multiplexer/demultiplexer 1, as shown in FIG. 1, for example, assume that the light having the wavelength λ1 (=1.31 μm) and the light having the wavelength λ2 (=1.55 μm) enter and propagate through the core 3. Then, these light are demultiplexed. Specifically, the light having the wavelength λ1 propagated through the core 3 and exits from the end surface thereof toward the thin-film filter 7 is transmitted through the thin-film filter 7. The light that has been transmitted through the thin-film filter 7 enters the core 4, and propagating through the core 4, exits from the end surface thereof. The light having the wavelength λ2 that has exited toward the thin-film filter 7 from the end surface of the core 3, on the other hand, is reflected on the thin-film filter 7. The light that has been reflected on the thin-film filter 7 enters the core 6, and propagating through the core 6, exits from the end surface thereof. The light having the wavelength λ1 which may be incident from the core 4, on the other hand, is transmitted through the thin-film filter 7, enters the core 3, and propagating through the core 3, exits from the end surface thereof.
In this optical multiplexer/demultiplexer 1, the light having the wavelength λ1 incident from the core 4 and transmitted through the thin-film filter 7 may also propagate through the core 6 by circumvention. The light of the wavelength λ1 that has circumvented into the core 6 constitutes a noise in the core 6 or causes a signal loss in the core 3, thereby posing the problem of a communication interference.
To suppress the circumvention of the light described above, a method is generally known as effective in which as shown in FIG. 2, the angle θ (hereinafter referred to as the branching angle) at which the cores 3 and 6 branch from each other is increased to increase the isolation between the cores 3 and 6. With the increase in the branching angle θ, however, the incidence angle (=θ/2) of the light entering the thin-film filter 7 also increases, and therefore the difference of the cut band between the P wave and the S wave is also increased due to the characteristics of the thin-film filter 7. In the case where the filter demultiplexing characteristic (transmission loss) of the low-pass thin-film filter 7 is shown separately for the P polarized light and the S polarized light, for example, as shown in FIG. 3, the area where the transmission loss undergoes a sudden change is different for the P polarized light and the S polarized light (polarization dependency). This deviation is expressed as the P-S wavelength difference Δλ in terms of wavelength. FIG. 4, in which the abscissa represents the branching angle θ between the cores 3 and 6 and the ordinate the P-S wavelength difference Δλ, shows the relation between the branching angle θ and the P-S wavelength difference Δλ. As shown in FIG. 4, the larger the branching angle θ, the larger the P-S wavelength difference Δλ.
As described above, with the increase in the difference of the cut band between the P and S waves, part of the S polarized light of the wavelength λ1 is reflected on the thin-film filter 7, or part of the P polarized light of the wavelength λ2 is transmitted through the thin-film filter 7, thereby separating the light rays. As a result, the optical signal is changed and the reproducibility is reduced. To prevent this situation, the upper limit value of the branching angle θ is required to be set by the P-S wavelength difference Δλ. Specifically, as shown in FIG. 3, let the wavelength band of the light having the wavelength λ1 of 1.31 μm be 1.26 to 1.36 μm, and the wavelength band of the light having the wavelength λ2 of 1.55 μm is 1.48 to 1.58 μm. Then, the distance between the two wavelength bands is given as 0.12 μm (=1.48−1.36). To prevent the P and S waves from being separated from each other, therefore, the P-S wavelength difference Δλ is required to be 120 nm or less. As seen from FIG. 4, the branching angle θ corresponding to the P-S wavelength difference Δλ of 120 nm is about 80 degrees, and therefore the branching angle θ is required to be about 80 degrees or less. Taking variations into consideration, however, the suitable branching angle θ is considered about 60 degrees or less. In the optical multiplexer/demultiplexers in general, the branching angle θ is desirably set to a maximum value not larger than about 60 degrees. Especially, the branching angle θ of about 60 degrees is considered appropriate to achieve a high isolation without increasing the polarization dependency.
In the optical multiplexer/demultiplexer 1 having the conventional structure, however, an increased branching angle θ to increase the light isolation makes it necessary to curve the considerably tilted core in the direction parallel to the length of the optical multiplexer/demultiplexer 1 resulting in an increased length along the curved portion of the core. A larger radius of curvature of the core, on the contrary, could shorten the length along the core. An excessively increased radius of curvature of the core, however, would increase the light leakage from the core. An attempt to suppress the light leakage, therefore, would increase the length along the curved portion of the core for an increased length and an increased width of the optical multiplexer/demultiplexer 1, thereby leading to a greater size of the optical multiplexer/demultiplexer 1. FIG. 5 is a diagram showing the relation between the branching angle θ between the cores 3 and 6 and the waveguide size (area) of the optical multiplexer/demultiplexer 1. As apparent from FIG. 5, the increase of the branching angle θ to about 60 degrees considered as the optimal value exponentially increases the waveguide size of the optical multiplexer/demultiplexer 1. In the conventional optical multiplexer/demultiplexer 1, therefore, a high isolation and a smaller size of the waveguide cannot be attained at the same time, and it is impossible to produce a small optical multiplexer/demultiplexer having a high isolation characteristic. Also, an increased waveguide size to secure a high isolation increases the signal loss due to the material loss of the core.